Thou wilt keep him in perfect peace, whose mind is stayed on thee: because he trusteth in thee.  Isaiah 26:3

Allow the Light of God'd love,  to take away the fear.  When hear what if, in our mind.  If it brings fear or doubt, it is not the voice of God.  We may need to change our thoughts like we do our clothes.  Replace the haunting thoughts with good thoughts.

If we have done something we think we shouldn't, after we have talked to God about it, if it  comes back to make us feel bad, change the thought to "Thank you Lord for loving me anyway."

Refuse to think on it.

If we are haunted by a possible illness. Change the thought to "If God allows me to walk there, He'll see me through."  It will take away the fear and the enemy's power over us.

Even Jesus had to refuse to listen to doubt.
 

But when he had turned about and looked on his disciples, he rebuked Peter, saying, Get thee behind me, Satan: for thou savourest not the things that be of God, but the things that be of men. Mark 8:33

The devil has no power unless we give it to him.  A cat sneaks up on it's prey, but the devil tries to scare us so he can devour us.  Be sober, be vigilant; because your adversary the devil, as a roaring lion, walketh about, seeking whom he may devour:  1 Peter 5:8